1. Field of the Invention
The present invention relates to a control system for a vehicle with a differential restricting device in particular, to a control system for a vehicle such as a four-wheel drive vehicle having differential restricting devices.
2. Description of the Related Art
A four-wheel drive vehicle is in general provided with a center differential mounted between a front propeller shaft and a rear propeller shaft for compensating the differential between the front wheels and the rear wheels, and a rear differential mounted between right and left rear wheels.
For example, Japanese Patent Laid-Open No. 62-166,114 discloses a four-wheel drive vehcle which comprises a front differential, a center differential, and a rear differential each of the differentials including a differential restricting device such as a clutch. The differential restricting device can be operated to be in locking condition or unlocking condition based on the various traveling conditions of the vehicle such as traveling on rough road, traveling in a straight line, an accelerating condition and a braking condition. The traveling conditions are determined by data representative of wheel rotating speeds and steering angles. The stability, braking performance, acceleration performance and the like of the vehicle can be improved by the such operations.
Japanese Patent Laid-Open No. 62-261,538 discloses a four-wheel drive vehcle with a center differential restricting device which is operated so as to restrict a differentiaI between the front wheels and the rear wheels based on an increase in the differential rotating number between the front wheels and the rear wheels. As a result, a good starting performance can be obtained when the vehicle is trying to get out of rough road.
Japanese Patent Laid-Open No. 63-57,332 discloses a four-wheel drive vehcle with a center differential restricting device which is operated based on the throttle valve opening and the vehicle speed so that tire abrasion and excessive fuel comsumption can be prevented when the vehicle is traveling at a high speed.
Japanese Utility Model Laid-Open No. 61-17,4340 also discloses a four-wheel drive vehcle with a center differential restricting device which is operated based on the throttle valve opening (the engine output).
In the above mentioned conventional four-wheel drive vehicle, the control operation of the differential restricting device based on the differential rotating number between the front wheels and the rear wheels means that the operation is carried out based on the slips which actually occur. On the other hand, the control operation of the device based on the throttle valve opening means that the control operation is carried out in order to prevent the slips before they actually occur.
However, since the actual slips occur differently in respective driving conditions, the above conventional four-wheel drive vehicle can not obtain a good performace in any driving conditions.